Facts
The appellants, [A.A.] and [B.A.] , are owners of parcel No. 106 of the land register of Liestal in the residential and commercial zone WG3. The parcel is built on in its southern area with three houses and a underground garage ramp; in the northern area there is a larger unbuilt area. It borders the Ergolz to the west, to the north an approximately 170 m long green space of the non-building zone in the municipality of Füllinsdorf, and to the east the Fraumattstrasse, beyond which lies the agricultural zone.
The appellants (building applicants) applied on 29 February 2016 to the Building Inspectorate Basel-Landschaft for approval of an apartment building and a playground in the northern area of the parcel. The City of Liestal lodged an objection. An e-mail from the head of the Cantonal Planning Department dated 9 March 2018 noted that the project lay almost entirely within the watercourse area of the Ergolz pursuant to the transitional provision of the WPO and could only be approved with an exception permit under Art. 41c WPO, which presupposed a densely built-up area. It was a borderline case; judicial confirmation could not be guaranteed. In addition, the playground would have to be raised because of the flood risk.
After several plan amendments, the Building Inspectorate rejected the building application on 10 September 2019, as the requirements for an exception permit under Art. 41c para. 1 WPO were found not to be met. The Building Appeals Commission Basel-Landschaft dismissed the appeal on 21 January 2020, as did the Cantonal Court Basel-Landschaft by judgment of 26 May 2021 following an on-site inspection.
Before the Federal Supreme Court, the appellants request that the cantonal court judgment be set aside and the building application be remitted to the building authority. The City of Liestal requests dismissal. The Federal Office for the Environment (FOEN) considers the refusal of the exception permit to be compliant with federal law. The Cantonal Court and the Federal Office for Spatial Development waived submission of observations.
In dispute are in particular the bed width of the watercourse and the width of the transitional watercourse area (16 m or 20 m), the qualification as a densely built-up area, the existence of an infill gap under Art. 41c para. 1 lit. a bis WPO, and a claim to protection of legitimate expectations based on an alleged change of practice and official assurances.
Considerations
The public law appeal under Art. 82 ff. FSCA against the final cantonal decision in building law is in principle admissible; the appellants, as building applicants, have standing under Art. 89 para. 1 FSCA. The Federal Supreme Court examines the complaints under Art. 95 FSCA and is bound by the facts established by the lower court (Art. 105 para. 1 FSCA), unless they are manifestly incorrect or arbitrary within the meaning of Art. 97 para. 1 FSCA; appellate criticism is inadmissible.
The transitional watercourse area is governed by Art. 36a WPA and the transitional provisions of the WPO, under which, for a bed up to 12 m, a strip of 8 m on each side plus the bed width applies. Whether the bed is 8 m or 12 m and the watercourse area thus measures 16 m or 20 m may be left open, since the building project protrudes considerably into the watercourse area even at 16 m and therefore requires an exception permit.
Under Art. 41c para. 1 lit. a WPO, zone-conforming installations may be approved in the watercourse area in densely built-up areas. According to settled practice (BGE 143 II 77; BGE 140 II 428; BGE 140 II 437), this is justified only where the watercourse area can no longer fulfil its natural function even in the long term, typically in densely built urban districts or village centres; in peripheral areas there is regularly no overriding interest in densification. The Cantonal Court denied the existence of a densely built-up area, since fewer than half of the buildings along the Ergolz lie in the watercourse area, the building parcel is peripherally located at the edge of the settlement area, and to the north there is an undeveloped green space of approximately 170 m. The FOEN confirmed this. The Federal Supreme Court found the concordant assessments convincing; even if a densely built-up area were assumed to the south, an infill gap to be closed towards the green area would in any event have to be denied (comparison Altendorf, BGE 143 II 77). Extending the on-site inspection to the largest perimeter was not necessary.
Under Art. 41c para. 1 lit. a bis WPO (in force since 1 May 2017), an infill gap within a row of built-up parcels may also be approved outside densely built-up areas where the watercourse area is…